This invention relates to a coating for paper, a coated paper sheet and a printed paper sheet. More specifically, the coating includes a combination of pigments and binders that lead to improved dry ink times. The sheet is useful for printing using offset, laser and ink jet techniques, including multifunctional printing.
Traditionally, commercial printing presses printed catalogues, brochures and direct mail using offset printing. Addressing of mailings was accomplished in a separate step using labels that were printed with an ink jet or a laser printer. In yet another step, the labels are applied to the offset printed product. By this multi-step process, specialized papers could be used for each different printer, taking advantage of the benefits of each.
Computerization of high speed printing operations has led to convergence of printing techniques by using multiple print devices on the same paper stock. Common parts of a document are printed using techniques for high speed or high quality, then personalized using a different technique. A brochure may be printed on an offset press, with non-image areas left in strategic places for individual information, such as the recipient's name and address, organization, custom products or pricing. The spaces are then imprinted with an ink jet or laser printer with the individualized information. However, conventional papers designed for offset printing may not function well for other printing techniques, such as laser or inkjet printing.
Papers used for ink jet printing must absorb droplets of ink rapidly to perform well in commercial high-speed presses. Aqueous inks are preferred for most applications due to a higher number of droplets per inch. Individual droplets should be absorbed preferably in less than 2 seconds, with the paper absorbing a certain amount of water to prevent ink smearing. Many standard paper grades manufactured for offset printing do not meet this requirement. For example, coating formulations containing more than 50 parts clay per hundred parts dry pigment, more than 5 parts starch, more than 15 parts latex or high binder concentrations in any region of the sheet would not meet the ink jet dry time requirement. Moreover, the carrier absorption must be uniform over the paper surface to assure even tones. Areas of high absorbency absorb more liquid than low absorbency areas, resulting in a mottled or blotchy appearance. This tendency is not detected by porosity measurements, which are averages over segments of the paper surface.
Laser printing applications perform better with a paper base sheet that acts as a thermal insulator. The surface must become sufficiently hot to fuse dry toner to the paper. If the paper conducts heat too readily, the surface will not become sufficiently hot to allow the toner to melt and produce a good image. Coatings for laser printing processes are formulated to hold heat near the surface of the base sheet.
Ideal papers for offset printing must absorb fountain solution from the emulsified ink or from a non-image area of the offset printing plate as well as the ink vehicle, usually oil. Offset printing is commonly used for color printing which requires multiple ink layers. The ink vehicle is absorbed at a controlled rate, known as the tack rate, such that enough oil is absorbed to set the ink, but leaves the ink sufficiently tacky to accept the next layer of ink as the paper continues through the press.
It has come to light that pigmented ink jet inks are particularly difficult to absorb evenly, especially on cationic coatings. When pigmented inks are absorbed, the pigments are held out on the coating surface, often blocking the pores so that it is difficult to develop wet resistance. Colors that are blended using several ink passes can require three or more ink coatings. If the pigments from the first two passes block the pores, the ink vehicle is not absorbed on the third and subsequent passes. This results in a poor image and inks that run in the presence of water.
Due to the many and varied demands on the paper substrate, no commercial paper is known that performs well for offset, ink jet and laser printing techniques. In order to make a coating and a printing paper that produces good print quality when printed by multiple processes, the coating must rapidly absorb water and ink oil, each at an appropriate rate, and retain heat at the surface of the sheet.